1. Field of the Invention
The invention relates to an alpha-beta titanium-base alloy having a good combination of strength and ductility, achieved with a relatively low-cost alloy composition. The invention further relates to a method for hot-working the alloy.
2. Description of the Prior Art
Titanium-base alloys have been widely used in aerospace applications, primarily because of their favorable strength to weight ratio at both ambient temperature and at moderately elevated temperatures up to about 1000.degree. F. In this application, the higher cost of the titanium alloy compared to steel or other alloys is offset by the economic advantages resulting from the weight saving in the manufacture of aircraft. This relatively high cost of titanium-base alloys compared to other alloys has, however, severely limited the use of titanium-base alloys in applications where weight saving is not critical, such as the automobile industry. In automotive applications, however, utilization of titanium-base alloys would lead to increased fuel efficiency to correspondingly lower the operating cost of motor vehicles. In this regard, two conventional titanium-base alloys, namely Ti-6Al-4V and Ti-6Al-2Sn-4Zr-2Mo, have been used in automotive engines designed for racing cars with excellent results. Specifically, the former alloy has been used in these applications for connecting rods and intake valves, and the latter alloy has been used for exhaust valves. In these applications, however, efficiency and performance are of primary concern with material costs being secondary.
Some of the factors that result in the higher cost of titanium-base alloys, such as the cost of the base metal, cannot at present be substantially changed. Factors that are subject to beneficial change from the cost standpoint are the cost of the alloying elements. Specifically, with the conventional Ti-6Al-4V alloy, the vanadium adds significantly to the overall cost of the alloy. Specifically, at present vanadium (a beta stabilizer) costs approximately $13.50 per pound and thus adds about 50.cent. per pound to the cost of the alloy. Consequently, if a less expensive beta stabilizing element could be used, such as iron, which costs about 50.cent. per pound, this would add only about 2.cent. per pound to the alloy if present in an amount equivalent to vanadium. In addition to the relatively high cost of vanadium, this is an element that is only obtainable from foreign sources.
Another factor that is significant in lowering the overall cost of titanium-base alloys is improved yield from ingot to final mill product. This may be achieved by improvements in mill processing, such as by reducing the energy and time requirements for mill processing or by an alloy composition that is more tolerant to current processing from the standpoint of material losses from surface and end cracking during mill processing, such as forging, rolling and the like. From the standpoint of increased yield from more efficient mill processing, an alloy composition that may be processed from ingot to final mill product at temperatures entirely within the beta-phase region of the alloy would provide increased yield because of the higher ductility and lower flow stresses existent at these temperatures. Consequently, processing could be achieved with less energy being used for the conversion operations, such as forging and hot-rolling. Currently, alpha-beta titanium-base alloys typically receive substantial hot-working at temperatures within their alpha-beta phase region. At these temperatures, during hot-working significant surface cracking and resulting higher conditioning losses result.
SUMMARY OF THE INVENTION
It is accordingly a primary object of the present invention to provide a titanium-base alloy having a combination of mechanical properties, namely strength and ductility, comparable to conventional alloys, including Ti-6Al-4V, at a relatively low cost alloy composition.
It is a further object of the present invention to provide an alloy of this character that can be hot-worked solely at temperatures above the beta transus temperature of the alloy to result in additional cost savings.
Broadly, in accordance with the invention, an alpha-beta titanium-base alloy is provided having a good combination of strength and ductility with a relatively low-cost alloy composition. The alloy consists essentially of, in weight percent, 5.5 to 6.5 aluminum, 1.5 to 2.2 iron, 0.07 or 0.08 to 0.13 silicon, and balance titanium. Optionally, the alloy may be restricted with regard to the oxygen content, with oxygen being present up to 0.25%. It has been determined that oxygen lowers the ductility of the alloy and thus is beneficially maintained with an upper limit of 0.25%. Particularly, oxygen contents in excess of 0.25% result in a significant adverse affect on ductility after creep exposure of the alloy of the invention.